One type of a conventional constant current circuit comprises a starter circuit and a constant current generating circuit. The starter circuit includes, for instance, first to third P-MOS transistors and first to third N-MOS transistors. In the starter circuit, the first and second P-MOS transistors are connected at sources to a power supply, and at gates to each other, and the first N-MOS transistor is connected at a drain and a gate to a drain of the first P-MOS transistor. The second N-MOS transistor is connected at a drain to a source of the first N-MOS transistor, and at a gate to the drain of the first P-MOS transistor. Furtheremore, the third N-MOS transistor is connected at a drain to a drain of the second P-MOS transistor, and at a source to the ground potential along with a source of the second N-MOS transistor and a drain of the third P-MOS transistor. The third P-MOS transistor is connected at a gate to the drains of the first P-MOS and N-MOS transistors, and at a source to the constant current generating circuit. The constant current generating circuit includes first and second diodes, first and second P-MOS transistors, first and second N-MOS transistors, and a resistance for deciding a circuit current. In the constant current generating circuit, the first and second diodes are connected at anodes to the power supply, and the first and second P-MOS transistors are connected at sources to a cathode of the first diode and through the resistance to a cathode of the second diode, respectively, and at gates to each other. Furthermore, the first and second N-MOS transistors are connected at drains to drains of the first and second P-MOS transistors, respectively, at gates to each other, and at sources to the ground potential.
In operation, when a voltage of the power supply is risen, the first and second P-MOS transistors are first turned on in the starter circuit, and the first and second N-MOS transistors are then turned on therein. Then, the third P-MOS transistor is turned on in the starter circuit to drive the first and second P-MOS transistors in the constant current generating circuit. Thus, the first and second N-MOS transistors are turned on in the constant current generating circuit, so that a constant current is supplied from a common connecting point of the drains of the first P-MOS and N-MOS transistors and the gates of the first and second N-MOS transistors. A value I of the constant current is defined by an equation (1), EQU I=(V.sub.D1 -V.sub.D2)/R (1)
Where V.sub.D1 is a voltage drop across the first diode, V.sub.D2 is a voltage drop across the second diode, an R is a value of the resistance.
Where the constant current I is supplied to a gate of the third N-MOS transistor in the starter circuit, a different value of a constant current is obtained from a connecting point of the drains of the second P-MOS transistor and the third N-MOS transistor.
However, the conventional constant current circuit has a disadvantage in that a starting voltage of operation is large, because the second diode, the resistance, the first P-MOS transistor, and the first N-MOS transistor are connected in series between the power supply and the ground potential in the constant current generating circuit.
Another type of a conventional constant current circuit comprises a P-MOS transistor, a serial connection of a diode and a Zener diode, first and second N-MOS transistors, and a load resistance. In the constant current circuit, the P-MOS transistor is connected at a source to a power supply, and at a drain and a gate to each other, and the diode is connected at an anode to the drain and the gate of the P-MOS transistor, and at a cathode to a cathode of the Zener diode which is connected at an anode to the ground potential. Furthermore, the first N-MOS transistor is connected at a drain to the power supply, at a gate to the gate and the drain of the P-MOS transistor and the cathode of the diode, and at a source through the load resistance to a drain and a gate of the second N-MOS transistor which is connected at a source to the ground potential.
In operation, when a voltage of the power supply is risen, the P-MOS transistor is turned on, so that a constant voltage of approximately 7.5 .+-.0.3 V is obtained across the serial connection of the diode and the zener diode. The constant voltage is applied to the gate of the first N-MOS transistor, so that a constant current is supplied from a connecting point of the load resistance and the drain and the gate of the second N-MOS transistor.
However, the latter conventional constant current circuit has also a disadvantage in that this circuit can not be adapted to a circuit comprising a comparator, a characteristic of which depends largely on a balance of threshold voltage values of P- and N-MOS transistors included therein, because a constant current characteristic depends solely on threshold voltage values of N-channel transistors due to the circuit structure including the first and second N-MOS transistors and the load resistance, in which a constant current is decided.